Maximun Speed

peterjaw

peterjaw

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Good day, captains,
My boat is Quicksilver 855 weekend, she has a Mercury Verado 300 outboard engine with a 15.3 x 14 propeller.
When I tested the maximum speed, the fuel tank is full (400 L), the water tank is full (100 L), and 4 persons (350Kg, roughly)
aboard.

The best speed I had was about 31 ~ 32 Kn while the engine's rev was about 5600 rmp. If I cut the throttle to about 2/3,
the rev was about 4200 rmp and the speed was about 20 kn.

However, a performance sheet I got from internet, it says my boat should have 37 ~ 38 kn WOT and 28.9 kn cruise.
On the sheet, it says 2 persons on board and full fuel and safety gears.

I use the same propeller as the performance sheet indicated.

I wonder what could cause such a big difference?

Thank you very much for advices and suggestions.
 
I don’t think I would be attempting a max speed run if I had a brand new engine.

Verado 300 wide open throttle is listed as 5200-6000rpm, so you’re in the right zone.
An engine with a few more hours on it, two fewer people onboard, maybe less water in the tank, and engine trimmed up a few degrees could help unleash the last few hundred rpm.

http://www.touron-nautica.com/pdfs/datos_rendimientoQS.pdf


.
 
Last edited:
For configuring the ActiveTrim (from Mercury) and also selecting the propeller, we need to sqeeze out the last rev.

I just sat in the cockpit and the dealer performed the test, so I don't know how the engine was trimmed.
=========================================================================================
To FlowerPower,
Yes, the link you provided was the data sheet I mentioned. Thank you.
 
If the wide open throttle on your engine should be between 5200 and 6000 (according to Flowerpower) even though you are bang in the middle at 5600. If you want maximum speed then you need to find those extra 400 revs, at the top end they make a big difference.
Best ways are, a clean prop and boat Hull, correct trim, lighter load (or the load is better trimmed ie people need to sit further forward) or different prop.
The speeds you report are fast. And not everything from the internet is true
 
Hi Peter
We have a 300hp Verado with a 4 blade Vensura prop 19” at just under 6000rpm we are getting 40/42kn. We mostly run at 4500rpm 27to30kn & burning 50ltrs per hour.
Boat Parker 750cc:
Length overall 7.46 m
Weight without engine ca. 1.860 kg
Beam overall 2.50 m

All the best CS.
 
Looking at the likely ratio on that verado...I’d have thought the wot rpm with a 14” would have been much higher at wot and trimmed out. Unusual pitch size too I’d think. Generally the props for this size gearcase come in odd numbers. With the exception of specialised stainless Props like an enertia. Are you certain of the size of prop ?
Also worth noting that prop selection is a mine field of variables. A real black art.
Be good to get some real definitive numbers under test.
 
Bouba said:
If the wide open throttle on your engine should be between 5200 and 6000 (according to Flowerpower) even though you are bang in the middle at 5600. If you want maximum speed then you need to find those extra 400 revs, at the top end they make a big difference.
Best ways are, a clean prop and boat Hull, correct trim, lighter load (or the load is better trimmed ie people need to sit further forward) or different prop.
The speeds you report are fast. And not everything from the internet is true
Click to expand...

Hi there. Worth noting that he doesn’t need to be at max rpm rating. Just in the range and as you might be getting at, usually ideally near the top. But In this case, if all the stated figures are accurate and given his concern around low speed. He will loose more speed by trying to get to 6000rpm (if figures are accurate) by means of a lower pitch prop. Likely lower by 2”. I’m pretty sure something isn’t right on this. The variables to beconsidered are vast. I’m perhaps thinking for starters another run needs to be done and record the figures from smartcraft data. Also I question the 14” prop size. Certainly not a standard size for that engine and set up. I would imagine it to be an odd number of pitch in standard form and likely of a higher pitch with that gear case and likely ratio.
 
Bouba said:
If the wide open throttle on your engine should be between 5200 and 6000 (according to Flowerpower) even though you are bang in the middle at 5600. If you want maximum speed then you need to find those extra 400 revs, at the top end they make a big difference.
Best ways are, a clean prop and boat Hull, correct trim, lighter load (or the load is better trimmed ie people need to sit further forward) or different prop.
The speeds you report are fast. And not everything from the internet is true
Click to expand...

Hi Bouba,
Mercury dealer told me that current condition is very good and good enough. I understand that not everything is tru on internet, but
the data sheet I got was from a 3rd. party (maybe a boat magazine's testing). If this data was from either Quicksilver or Mercury, then I
will probably give it 20% off.

As you know that my boat is brand new, so I guess the only factor is the propeller. I guess the Mercury dealer just don't want to do
the trial-n-error job so ......

On the other hand, you are right, to pursue the extra 400 revs may not be that important.
 
cstewart37 said:
Hi Peter
We have a 300hp Verado with a 4 blade Vensura prop 19” at just under 6000rpm we are getting 40/42kn. We mostly run at 4500rpm 27to30kn & burning 50ltrs per hour.
Boat Parker 750cc:
Length overall 7.46 m
Weight without engine ca. 1.860 kg
Beam overall 2.50 m

All the best CS.
Click to expand...
Thank you very much, CS.

This information is very useful, I will write it down.
 
QBhoy said:
Hi there. Worth noting that he doesn’t need to be at max rpm rating. Just in the range and as you might be getting at, usually ideally near the top. But In this case, if all the stated figures are accurate and given his concern around low speed. He will loose more speed by trying to get to 6000rpm (if figures are accurate) by means of a lower pitch prop. Likely lower by 2”. I’m pretty sure something isn’t right on this. The variables to beconsidered are vast. I’m perhaps thinking for starters another run needs to be done and record the figures from smartcraft data. Also I question the 14” prop size. Certainly not a standard size for that engine and set up. I would imagine it to be an odd number of pitch in standard form and likely of a higher pitch with that gear case and likely ratio.
Click to expand...

Hi QBhoy,
You are right, beside the maximum speed, I also wonder why my cruise speed is much slower. I was expecting 24 ~ 26 kt but only
got 18 ~ 19 kt.
 
Air temperature is crucial, the cooler the air the more dense it is, the more dense it is, the more fuel can be injected and the more power and engine revs you can get. According to the specifications I read the ambient testing temperature was the standard 20 degrees Celsius. Now you know why your boat goes faster in a cold winter.

On an F1 car the power increases can be as much as 70-85 BHP by way of comparison.
 
You are right, Assassin. You just remind me about the imfluence from temperature. The same situation happens to my car.

I did not notice the testing temperature was 20 C. My condition is 35 C and humidity higher than 75%. I guess the 15C difference can
make a huge impact.
 
Not super convinced that a rise in air temp or sea temp will be hugely significant .
I have been riding around in 34/36 degree air temp and 28 sea temp and noticed very little drop off if any compared to winter 15 air and 15 sea .
Although diesels they have Sea Water Charge Air coolers and turbos + a load of electrotwackery - legitimate ( F1 reference ? )
The charge air temp delta seems the same so the SWCAC seem to be able to cope .
It will be the props , not just the pitch ,but rake and blade ratio .

Are you sure they are identical in every measurement ,and material ie Stainless steel or what ever .
Inferior stuff my distort under load and thus is not really the same dimensions when under load .

A cruise of 18/20 knots is the give away because people don,t race around at WOT .
While nice to know on any set up ,petrol or diesel it’s rarther academic , it’s the cruise figure that counts .

Check the numbers on the prop ,and report back
 
peterjaw said:
Hi Bouba,
Mercury dealer told me that current condition is very good and good enough. I understand that not everything is tru on internet, but
the data sheet I got was from a 3rd. party (maybe a boat magazine's testing). If this data was from either Quicksilver or Mercury, then I
will probably give it 20% off.

As you know that my boat is brand new, so I guess the only factor is the propeller. I guess the Mercury dealer just don't want to do
the trial-n-error job so ......

On the other hand, you are right, to pursue the extra 400 revs may not be that important.
Click to expand...

I probably didn’t make myself clear. If it’s was my boat I want those 400 revs.
I’ve only owned 90 hp and 150 hp outboard boats (not a monster like you have) but 400 revs at the top end can easily add a few knots.
 
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